Perhaps the most chilling piece of evidence released is security video that shows Aaron Alexis, 34, methodically moving through Building #197 armed with a sawed-off shotgun. During a press conference, Valerie Parlave, assistant director in charge of the FBI's Washington Field Office, said he was shooting at random people. We'll embed the video; it doesn't show Alexis shooting, but be warned some may still find it very disturbing:

Also of note: investigators found "electronic media" in a backpack Alexis left inside a bathroom at the Navy Yard.

The FBI is still analyzing the documents, Parlave said, but there are clues Alexis was motivated by a belief that he was being attacked by electromagnetic waves.

"There are multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by extremely low frequency (ELF) electromagnetic waves. The etching of 'My ELF weapon!' on the left side of the receiver of the Remington 870 shotgun is believed to reference these electromagnetic waves. In addition, a document retrieved from his electronic media stated, 'Ultra low frequency attack is what I've been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this.'

"ELF technology was a legitimate program for naval sub-tonal submarine communications; however, conspiracy theories exist which misinterpret its application as the weaponization of remote neural frequencies for government monitoring and manipulation of unsuspecting citizens."

Update at 2:21 p.m. ET. A Timeline:

The FBI also said that Alexis' shotgun was etched with several phrases: "End to the torment!" and "Not what yall say!" and "Better off this way!" and "My ELF weapon."

Investigators put together this timeline of the shootings:

7:53 a.m. - Alexis' rental car, a blue Toyota Prius with New York plates, entered Parking Garage #28 at the Washington Navy Yard, located directly across from Building #197.